The invention relates to a screw provided with self-locking, rolled thread, wherein, in a plurality of places along its extent, said thread is deformed in relation to the normal thread cross section in that the thread outside diameter is reduced by radial compression of limited thread portions, the area of the thread cross section being substantially identical to that of the undeformed thread.
Screws with self-locking threads are known in a variety of forms. For example, the turns of a male thread are, circularly about the thread root, bent away from the head end in the axial direction in order thereby to produce a special friction in relation to a female thread (DE-OS 2062342). DE-OS 2257112 describes a deformation, referred to as a rib, of the turn of a male thread, said rib extending over only a portion of a thread turn. In the case of a self-locking thread described in DE-PS 1575259, the flank angle is, circularly about the thread root, changed from thread turn to thread turn in the case of straight flanks. As is demonstrated by DE-PS 1750206, it has also already been proposed, circularly about the thread root, to increase the profile thickness of the locking region of the thread. Tending in the same direction is the proposal according to DE-PS 1500981, according to which the locking portion of a thread provides for an enlarged core diameter with reduced outside diameter.
Finally, mention should also be made of DE-PS 3743010, according to which, in the case of a male thread, the locking region is created over a plurality of thread turns in that, in the locking region, the thread crests are pressed radially inwards, there being in the cross section a rectangular design in the region of the thread crests, the overall volume of the thread thereby being maintained.
With the hitherto known screws with self-locking threads, it has been necessary, because of the design of the locking part, to accept that there will, to a considerable extent, be a plastic deformation, when the screw is screwed in, either of the female thread or of the thread of the locking part, this leading to difficulties, particularly in the case of hard materials for the screw or the female thread. The object of the invention, therefore, is to design a screw provided with a self-locking thread such that its limited thread portion serving the purpose of locking makes do with only slight deformation of the female thread and/or of the screw thread and the required resistance for producing a sufficiently high further turning moment is achieved through maximum-area contact between screw thread and female thread.
The object of the invention is achieved in that, while maintaining the slant of the thread flanks, the thread cross section is symmetrically widened in the outer region and reduced in the adjoining inner region, the transition from the widened to the reduced thread cross section lying approximately in the centre of the respective thread flank, and in that the limited thread portion extends over two parts of a thread turn such that there result two symmetrically opposite thread portions each of approximately ⅙ of the thread turn.
The compression of the thread turns in each region of the limited thread portion and the there resulting reduction of the outside diameter of the thread mean that, during the rolling of the screw, the necessary material is made available for the widening of the thread cross section in the outer region of the thread, said material then contributing particularly effectively to the aforementioned large-area contact in that region which is particularly effective in this regard, namely the outer region of the thread turns, where the greatest resistance to the further turning of the screw is particularly high owing to the there existing relatively large diameter. In order to achieve the desired locking effect to a high degree, only a relatively small increase of the thread cross section is required, depending on the hardness of the material for the screw or female thread, this making it possible, with regard to the deformation in the region of the limited thread portion or the opposite part of the female thread, to obtain only very small deformations in order to guarantee the required security. The aforementioned design, therefore, results, with relatively little effort as regards the rolling of the screw, in a high degree of security against further turning; that is, the screw has a high further turning moment.
In order to be able to influence the further turning moment of the screw as desired, the screw is advantageously designed such that the limited thread portion is provided in a plurality of spatially separate places on the screw, the region of the thread beginning being kept free from the limited thread portion. The latter design ensures that the screw is able initially to enter a female thread without resistance, as a result of which the screw is then guided and only then comes up against the limited thread portion for the locking of the screw. The greater is the number of such limited thread portions, the higher is then the further turning moment which must be exerted on the screw.
Advantageously, the thread turns with limited thread portions are provided directly adjacent to each other, with the result that, in this case, the further turning moment is increased correspondingly quickly.
In the case of adjacent thread turns with limited thread portions, said thread portions are advantageously offset by 90° in relation to each other from thread turn to thread turn, as a result of which the pressure on the female thread is offset; that is, pressure on the female thread at the same places with regard to angle, i.e. at opposite places, is avoided.